Polarographic Method for Estimation of Chromium in Ruby Crystal SIR: The ruby owes its pink color to an extremely small amount of chro-. mium, incorporated in the aluminum oxide structure. Its most recent application is in the preparation of laser devices. The amount (about 0.05%) and homogeneous distribution of chromium are two very important factors governing laser action in a ruby. In Verneuil's flame-fusion technique (4) for the production of rubies, =me degree of inhomcgeneity occurs during crystal growth, sariously decting its properties. No absolutely foolproof method is available for the synthesis of perfect ruby CmStak COEbffihg the d&Xd amoynt of chromium. A very simple and convenient halytical method for the estimation of chromium in ruby is needed, in order to asse9s its suitability for use in laser devices. Two excellent chemical methods (I ,9) involve the decomposition of a finely powdered sample by fusion methods and subsequent development of a chromiumdiphenyl carbaeide color which is estimated spectrophotometrically. The method described here replaces the colorimetric procedure by polapgraphy. ?he pwdered ruby Ytrmpie is converted by fusion into chromate, which is then estimated polarographically. EXPERIMENTAL
The extreme hardness of ruby creates a considerable problem in the preparation of an uncontaminated finely powdered sample. Ruby crystals are difficult to decompose except by certain fusion methods, for which the sample must be in a finely powdered state. Care is needed to avoid the introduction of extraneous chromium during crushing. Sampling. Small pieces of ruby are held in platinum-tipped tongs, heated to redness in a flame, and dropped into water. A coarse powder results on account of thermal shock. The powder is then crushed in a hardened steel mortar, then sieved through a 120-mesh sieve. The powdered sample is boiled with aqua regia to remove any contaminants, washed with deionized water, and dried nt 110' C. Fusion Mixture. A fusion mixture consisting of Anal r?. grede anhydrous Yodium carbonate and anhydrous sodium tetraborate (in the ratio 2 to l ) suggested by Chirnside et al. (I) h y been found suitable for the decomposition of the powdered ruby sample. The reagents were purified as suggested by these authors.
Table I.
Polarographic Characteristics of Chromate at D.M.E.
Temperature. 25' f 0.1' C. Ells = -0.76 volt against mercury pool anode. Suppyting electrolyte = 0.5N NaOH
7tl = d.99 mg./sec. t = 2.95 see; m * ! a p 6 = 3.012 mg.2" gc.-11*
Diffusion current measured at -0.85 volt
Sample no. 1
2 3 4 5
Chromate conm.,
1)iffusion
mmole, C
Diffusion current (wrr.),Ma., id
0.10
2.100
0.03 0.06
1 640 1.260
0.0: 0.02
0.421
current
conet unt, idlttP2" 1"' c dlC = K 21.00 6.971 2i;. 50 G .E M 21 .oo G . 971 20.75 0.8'30 21.05 6,987 Av. 20.84 =k 0.34 6.025 f 0.119
0.ti;jU
.-
mg.j than in the chemical method (2
to 10 mg.), since smaller quantities produce no wave of measurable height. However, it has been possible to achieve complete fusion of even greater sample quantities with sodium carbonate-sod i m tetraburate flux, and no special prsS!t-~iait siicctieisd. Polarography. A 0.1 to 1S SaOH supporting electrolyte medium was suggested by Lingme and Kolthoff (3) for the polarographic estimation of chromate ion, since a well defined wave with height directly proportional to chromate ion concentration is produced in this medium. Sone of the other elements found in ruby, such as aluminum, iron, etc., interfere with the procedure. Calibration. Polarograms of chromate ion have been recorded over a concentration range of 0.02 to 0.1 m M . A 0 . 5 S S a O H medium is employed as supporting electrolyte. Analytical reagent grade potawium chromate is used for pre aring standard chromate solutions. +he polarograms are recorded by a Cambridge pen wv1 king polarograph, with 2 ml. of standard solution placed in the polarographic cell and nitrogen being bubbled through the solution for about 10 m u t e s . The halfwave potential, capillary characteristics, and other observations are listed in Table 1. The calibration curve is obtained by plotting diffusion current (correztcd for diffusion current due to s u p porting electrolyte alone) against concentrations.
Table 11. . Amount of Chromium in Ruby
Samples Chromium, Sample no. I
Spectro-
photometric Polarogrdphic
3
0.03 0.6.38 0 043
4
0.047
2
0.029 0.0374 0 0427 0.0463
heated for 15 minutes in a muffle furnace at a temperature of 1oOO' C. The crucible wa5 cooled and transferred to a 250-ml. beaker to which 50 ml. of 0.4N sulfuric acid was added. The beaker was heated on a hot plate until the melt dissctlved. Then the crucible was removed and the solution contained in the beaker WRS evaporated to dryness. The dried m s s was boiled with 20 ml. of distilled water to dissolve it completely. The solution was transferred to a 50-ml. volumetric flask, 25 ml. of 1N NaOH solution was added, and the volume was mad(*up to the mark. The polarogram of n 2-ml. aliquot was recorded with a recorder sensitivity of 1/5. The diffusion current due to the ruby sample was rorrc.cted by subtracting from it the diffwion current due to the supporting elertrolyte alone.
Calculation. The diffusion current due to 2 ml. of solution 1 is found to be 0.143 pa, When this value L put into the equation id = KC
PROCEDURE
A finely powdered ruby sample (sample 1, 38.5 mg.) ww placed in a Wml. platinum crucible along with 400 The polarographic method s u f f e ~ mg. of sodium carbonate-sodium tetraborate flux. This was then mixed from one disadvantage, that it is required intimately and the covered crucible was to work with larger samples (30 to 50
where id = diffusion current C = concentration. millimoles
K
a
(4
aconstant h .-
--
the concentration of chromate in t,he VOL. 38, NO. 13, MCWYER 1966
1933
sample solution can be evaluated and the amount of chromium in the ruby sample calculated. The results obtained by spectrophotometric and polarographic methods for different ruby samples are listed in Table 11. When the polarographic method is applied to a ruby sample after converting its chromic oxide into chromate by fusion and then estimating it polarographically, the results obtained are in
reasonable agreement with those obtained by chemical methods. ACKNOWLEDGMENT
The author expresses deep gratitude to Kartar Singh, director, for suggesting the problem and his keen interest during the progress of the work and H. K. Acharya, assistant director, for providing the necessary facilities.
LITERATURE CITED
(1) Chirnside, R. C., Cluley, H. J., Powell, R. J., Profitt, P. M.C., Analyst 88, 851 (1963). (2) Dodson, E. M., ANAL.CHEM.34, 966 (1962). (3) Lingane, J. J., Kolthoff, I. M., J. Am. Chem. SOC.62, 852 (1940). (4) Verneuil, M. A,, Ann. Chim. Phys. 3, 20 (1904). S. S. SINGH
Chemistry Division Defence Science Laboratory Metcalfe House, Delhi-6 India
otentiometric Determination of Stabilities of Molybdenum(VI) ungste n (VI) Chelates SIR: Although hexavalent molybdenum and tungsten coordinate with a great number of ligands, only limited stability data exist for such complexes. Recent work in this laboratory on the structural and bonding characteristics of various Mo(V1)-aminopolycarboxylic acid complexes has led t o the evaluation of stability constants from proton nuclear magnetic resonance (K'MR) data (9, 3 ) . These studies have suggested the possibility of using potentiometric pH titration techniques to measure the stabilities of Mo(V1) and W(VI) complexes. One advantage of using potentiometric methods is that the solution ionic strength may be kept low and constant, a condition not possible using NMR techniques because of the high concentrations required. The ligand systems which have been studied are iminodiacetic acid (IDA), N-methyliminodiacetic acid (MIDA), nitrilotriacetic acid (NTA), and (ethylenedinitrilo) tetraacetic acid (EDTA). Comparisons are made of the potentiometrically determined constants among the ligands and between Mo(V1) and W(VI). These constants are also compared to those determined by NMR methods. I n the usual potentiometric method for evaluating metal-ligand stability constants, the competition between metal ion and hydrogen ion for the ligand is studied, and the pH region of interest is from about 1 to 5 (6). In the i\Io(VI) and W(V1) (hereafter indicated as just Mo and W) systems, however, the complication of metal polymerization is introduced in acidic solutions. Because the polymerization equilibria are not well understood, this pH region is not useful for stability determinations. In more alkaline solutions, on the other hand, a pH-dependent process involving the competition between molybdate or tungstate formation and metal-ligand complexation-Le., a competition between ON- and ligand for the metal ion-can be utilized. This process was
determined from the NMR studies to be important from about pH 6 t o 9 and can be represented by
where M represents either 310or W and L represents the aminopolycarboxylic acid ligand. I n the pH region above 6 no evidence was found for any Mo species containing fewer than three oxygen atoms-e.g., Mo02'2-as has been proposed for other systems (8). The molybdenum coordinating species in all the aminopolycarboxylic acid systems above pH 6 is Mo03, and by analogy we have assumed that the corresponding coordinating unit in the tungsten systems is WOa. EXPERIMENTAL
Reagents. Analytical reagent grade sodium tungstate dihydrate and molybdenum trioxide were used without purification. N-methyliniinodiacetic acid (Aldrich Chemical Co.) (ethylenedinitrilo)tetraacetic acid (Baker Chemical Co.), and nitrilotriacetic acid (Eastman Organics) were also used as received. Disodium iminodiacetate (Aldrich Chemical Go.) was twice recrystallized from aqueous solution before use. Stock solutions of 0.010M KTA and MIDA and of 0.150M Ka2TV04 were prepared determinately. A diacid solution (0.010M) of IDA was prepared by titrating a solution of disodium iminodiacetate with HC1. EDTA solutions were prepared by dissolving the requisite amounts of the solid metal chelates, Na4(Mo03)zEDTA.8Hz0 and Na4(W03)2EDTA.8Hz0,synthesized by the method outlined previously (4). Stock 0.150M Kzhf004 was prepared by titrating a slurry of MOO3 with KOH. A 1.OM KN03 solution was prepared using reagent grade KNOa which had been twice recrystallized from water. All solutions were made up with triply distilled water which was degassed with Nz before use. Potentiometric Titrations. Solutions to be titrated were 1.00 X 10-3M
in ligand, 0.0150M in metal (&$f.004 or N ~ ~ W O Iand ) , 0.10X in KN03. The titrant was 0.0931M KOH prepared from a 45% Cos-free KOH solution. The solutions for titration of the free ligands for determination of the acid dissociation constants mere 1.00 x 10-aM in ligand and 0.10M in KNOa. The solution p H was monitored a8 KOH was added using either a Corning Model 12 or a Leeds and Northrup line-operated pH meter, standardized before and after each titration with saturated potassium acid tartrate (pH 3.66) and 0.01X sodium borate ~ D H 9.18). All work was carried ou< a t 25 f 0.5' C. Equilibrium Calculations. The method used for determining the acid dissociation constants of the protonated ligands was exactly analogous to that outlined by Schwarzenbach (6). The method for calculating the metalligand formation constants was similar to that outlined for metal-EDTA titrations (6),but was modified to take into account the different species present. For the IDA, MIDA, and NTA ligand systems, NMR studies indicate that only one metal-ligand species exists above The following equapH 6, MOIL-.. tions and equilibria were considered in the IDA and MIDA systems:
+
+ L-' + HzO; K f
~1fo4-~ 2H+
MO&-2
HL-
s H*
+ L-2;
K, =
3
